Due to the enormous progress recently achieved in the solid-state lighting technology, light-emitting diodes (LEDs) are now approaching the luminous performance level of those provided by conventional light sources while offering substantially longer operational lifetime, superior lumen maintenance, and color-rendering property with improved reliability and handling safety compared to light bulbs and fluorescent tubes. Due to the very low operating voltage of LEDs, lamp drivers are relatively simple to design and more reliable due to smaller stresses on driver components compared to fluorescent lamps which require high starting voltage and precise control of heating current for filament both during the start-up phase and dimming action. Therefore, the use of LEDs is becoming more popular and is quickly replacing incandescent, halogen, and fluorescent lamps in residential, commercial, and industrial applications.
With LEDs being increasingly used in different illumination applications, the necessity for an efficient driver with an optimized control circuitry becomes more important. Since LEDs are current-driven devices in which light is produced via the recombination of injected holes and electrons in a semiconductor junction, the luminous intensity of LEDs is typically controlled by controlling the forward current flowing through the device. Since LEDs conduct current only in one direction, the default method for driving LEDs is by using a constant DC current, known as amplitude-mode driving technique. Nevertheless, it is found that the peak emission wavelength of LEDs tend to shift with the forward current which can lead to color variations at different luminosity levels. This problem imposes significant challenge when the LEDs are used for LCD backlighting where color stability is of primary importance. White light can be produced by combining the three primary colors each generated by individual LEDs, or by using phosphor conversion of blue or UV LEDs, all of which are prone to color variations when driven at different forward currents. This makes the amplitude-mode driving technique unsuitable for use in dimming LEDs as the forward current must be continuously adjusted for various luminosity levels. Even when operated without dimming, the small dynamic resistance of LEDs also imposes stringent requirement on the control accuracy of the DC current as ripple effect can also lead to the same color variation problem. Also, when driving multiple LEDs, the number of LEDs connected in series is limited by the output voltage available from the DC power source and often necessitates the use of step-up voltage conversion stage. For LEDs connected in parallel, unequal current sharing between the LED branches due to the manufacturing spread, aging and temperature variations also leads to spatial variations in the luminous intensity and color. Furthermore, one of the most attractive features of LEDs is their extremely long lifetime hence the LED drivers should also have a comparable lifetime. However, the electrolytic capacitors typically used at the output of DC power source have placed limitation on the lifetime of these drivers, especially when the drivers have to operate continuously under high ambient temperature such as inside the light fixture or close to the high-power LEDs.
Some of the problems discussed above can be eased by driving the LEDs with AC current. If the mains power is used directly as the power source, AC current can be supplied to the LEDs without further power conversion stage. This substantially reduces the cost and improves the lifetime of the LED driver due to the absence of electrolytic capacitor. Since LEDs are uni-polar devices, this type of driver should be used for driving LEDs arranged in anti-parallel (or full-bridge LED module) so that light is obtained at both half cycles of the AC current. However, as the LED current flowing through each branch is pulsating at 50 or 60 Hz, the light quality can be significantly degraded due to flickering. To eliminate this problem, high-frequency AC current is used but this requires an intermediate AC-AC power conversion stage which overwrites the advantage of a simple mains AC driver and makes other driving techniques more attractive, such as pulsating DC. With AC current, the LEDs can be dimmed by sampling the AC current at regular intervals through on-off switching at a substantially higher frequency than the fundamental AC current. Thus it is expected that driving LEDs with pulsating current conveniently achieves both the LED powering and dimming functions.
The technique of driving LEDs with pulsating DC current is more commonly known as the burst-mode or pulse-width-modulation (PWM) driving technique. This is a method by which the LED is switched on and off at high frequency and the luminous intensity is controlled by adjusting the duty cycle, the ratio of the time the device is on to the switching period, hence the average forward current. Since the peak current level is kept constant during the switching, the control of luminosity level by dimming is independent of the color thus the light chromaticity is improved. In practice, some ripples are often present in the peak current and it should be minimized so that the average driving current can be controlled precisely, especially when the LEDs are used as LCD backlight. Despite improved chromaticity and flexibility for dimming, the PWM drivers are inherently more complicated since a combination of DC power source (AC-DC or DC-DC converters) and switching network is usually needed, which increases the driver complexity. Fast transients produced by the switching of LED current also cause EMI problems, which must be overcome by additional EMC design at increased cost.
Among the three driving techniques discussed, amplitude-mode and PWM-mode driving are most commonly adopted by the LED industry. The interactions between the current waveforms used (DC and PWM) and the luminous characteristics of LEDs are rarely discussed. The luminous intensity of LEDs tends to saturate at high forward current. This feature gives rise to different luminosity levels when the LED is driven by DC and PWM currents of the same average value. Generally, for a given average forward current, amplitude-mode driving technique always produces a higher luminous intensity compared to the PWM-mode driving technique since the latter operates at higher peak current level where less light is produced due to the saturation phenomenon. When linearly averaged by the duty cycle, this results in a lower luminous intensity. However, with better chromaticity and flexibility for dimming, the PWM-mode driving technique is preferred and more often used in practice where dimming is required at the expense of poorer luminous efficacy.